The present invention relates to a single mode wide-band optical fiber coupler having reduced wavelength dependence of its splitting ratio, and a method of making such an optical fiber coupler.
A conventional single mode optical fiber coupler is produced by twisting together a prestretched optical fiber and an unstretched one, or by fusing together such optical fibers held in parallel juxtaposition with each other and then stretching their coupling region, as set forth in PCT Publication W087/00934 or Japanese Patent Kokai (or Application Laid Open) No. 108311/88 based on U.S. patent application Ser. No. 918,966 filed Oct. 15, 1986 (U.S. Pat. No. 4,798,438).
In the manufacture of a single mode optical fiber coupler having reduced wavelength dependence of its splitting ratio, the ratio between the diameters of the two optical fibers in the region where they are fused together determines the optical characteristics of the optical fiber coupler. It is therefore important to implement the above-said diameter ratio without scatter. However, since the diameter of a reduced-diameter portion 13 of a prestretched optical fiber 11 varies along its longitudinal direction as shown in FIG. 1, the diameter ratio between the diameters of the two optical fibers changes with the position where they are fused together.
The conventional manufacturing method disclosed in the above-mentioned PCT publication does not ever take into account a solution to such a problem, and hence is very low in the yield rate of product.
In the above-mentioned Japanese Patent Kokai, it is proposed as a solution to the problem to stretch the optical fiber or to reduce the diameter of the optical fiber by chemical etching so that the reduced diameter is uniform along the longitudinal direction of the fiber. It is very difficult, however, to draw the optical fiber while holding its diameter uniform lengthwise thereof. Moreover, even if the diameter of each optical fiber can be reduced uniformly over a certain length, it is also very hard to maintain the two optical fibers in parallel juxtaposition with each other. As shown in FIG. 2, the optical fibers each bend largely at both ends of their contact portions 13 disposed in parallel to each other, and this bending increases the transmission loss of the optical fiber coupler.
As mentioned above, the manufacture of an optical fiber coupler having reduced wavelength dependence of its splitting ratio calls for stabilizing the ratio between the diameters of the two optical fibers in the region where they are fused together. To this end, the prior art stretches each optical fiber or decreases its diameter by chemical etching so that the diameter is uniform over in its longitudinal direction. However, much difficulty is encountered in reducing the diameter of the optical fiber without changing the diameter lengthwise thereof i.e. the central portion of the reduced-diameter portion is smaller in diameter than its end portions, and consequently, it is very difficult to maintain the two optical fibers in parallel juxtaposition with each other.